Conventional portable illumination systems, such as flashlights, include an incandescent light bulb and conventional drycell batteries enclosed in a housing typically constructed of a body section and a head section. Though portable, illumination systems of this type are often bulky and cumbersome, and thus often are not carried on the person. This presents a problem when the need for illumination arises unexpectedly, such as during a power failure at night, and no portable illumination system is readily available. The problem is exacerbated when, for instance, the person does not have a hand available to search for, aim and operate a conventional flashlight.
There have been some variations on portable, hands-free illumination systems. Underground coal miners have long mounted large conventional incandescent lights with large, heavy battery packs to the front of hard hats, allowing them to work with both hands while the light on the hat illuminated the area in front of them. U.S. Pat. No. 3,032,647 to Wansky provides an example of a hat-mounted light. U.S. Pat. No. 4,616,297 to Liu also provides an example of a hands-free flashlight, which mounts conventional incandescent light bulbs and conventional drycell batteries to a bulky frame to be worn like spectacles. U.S. Pat. Nos. 5,217,294 to Liston and U.S. Pat. No. 5,412,545 to Rising describe hands-free operation of a conventional flashlight by strapping the flashlight to the user's head.
While any of these devices, if worn by a user, might function as a hands-free illumination device, such devices are even more bulky and cumbersome than conventional flashlights. Moreover, such devices are not suitable for use in conjunction with other headgear.
Conventional hands-free portable illumination systems are bulky primarily because they use incandescent light bulbs, which suffer from poor efficiency in converting electrical power into radiated visible light. Most of the electrical energy incandescent light bulbs consume is wasted in the form of heat energy, while less than 7% of the energy they consume is typically radiated as visible light. This has severe negative consequences for portable illuminator applications, such as hands-free illumination devices, where the amount of power available for lighting systems is limited. In these applications, electrical power is usually provided by batteries which are periodically replaced, as in the case of a flashlight. Such a mechanism for providing electrical power is inherently bulky, heavy, and/or expensive due at least in part to poor power-conversion efficiency in generating visible light.
As already noted, incandescent lamps generate large amounts of heat for an equivalent amount of generated light as compared to other sources. This results in very high bulb-wall temperatures typically in excess of 250 degrees Celsius and large heat accumulations which must be dissipated properly by radiation, convection, or conduction to prevent damage or destruction to the illuminator support members, enclosure, optics or to other nearby components. Providing room for the necessary heat dissipation requires additional bulkiness. This high heat signature of common incandescent light sources in illuminators is particularly problematic for hands-free light sources, where the light source is in close proximity to the user's body, for instance, the user's head.
Incandescent light bulbs, or lamps, are also disadvantageous because they are fragile and have a short life. Even in stable environments incandescent lamps must be replaced frequently, sometimes at great inconvenience, hazard, and/or expense. In addition to their short life, incandescent lamps are susceptible to damage from mechanical shock and/or vibration. Such occurrences can damage the delicate filaments from which incandescent light emissions originate, or can damage the surrounding glass casing, which can result in air entering the casing and quick burning out of the filament. Incandescent lamps can also be easily damaged by exposure to liquid moisture, due to the thermo-mechanical stress associated with contact between the hot glass bulb wall and cooler fluids. Thus, incorporating an incandescent lamp into a hands-free illumination device requires substantial or extreme measures to protect the light bulb from shock, vibration, moisture and other hazards while still allowing for removal of the bulb or light fixture when it burns out, is permanently damaged or otherwise loses power.
Incandescent light bulbs or lamps also exhibit certain electrical characteristics which make them inherently difficult to incorporate in small, lightweight applications, like hands-free illumination devices. For instance, when an incandescent light source is first energized by a voltage source, there is an initial surge of current which flows into the filament. This inrush current, which is typically 12 to 20 times the normal operating current, limits the lifetime of the lamp thus further amplifying the need for an illuminator structure which allows for frequent replacement. Inrush current also necessitates unusual consideration when designing supporting electrical circuits which contain them. Fuses, relays, mechanical or electronic switches, wire harnesses, and connectors electrically connected to such lamps must be capable of repeatedly carrying this extreme transient.
In addition, the voltage-current (V-I) characteristic of incandescent lamps is notoriously non-linear, as are each of the relationships between light output and voltage, current, or power. The luminous intensity, color temperature, and service life of incandescent lamps varies exponentially as a function of applied current or voltage. This sensitivity to power source variation makes electronic control of incandescent lamps a particularly difficult problem. They are further susceptible to significant reliability and field service life degradation when subjected continuously to DC electrical power, pulse-width modulated DC power, simple on/off switching of any sort, or any over-voltage conditions, however minor. Incandescent lamps also possess significant inductance which, when combined with their relatively high current load, complicates electronic switching and control greatly due to inductive resonant voltage transients.